tools/env: Use a board-specific default env

Originally added in aa701b94

Before this patch, there was a hard-coded env that was used as default
if the env in flash is detected as invalid.  Now this tool (compiled
for a given board) will share the default env with the u-boot for the
board.

Fix include of config.h

Need to define "TEXT_BASE" when building the fw_env tool so that the
default env will be correct for environments which use it.

Define __ASSEMBLY__ when calling #include <config.h> so that we only
get #defines (all we're interested in).

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>